This invention relates to a method of assembling a cathode ray tube bulb assembly and mount assembly, and particularly to a method of assembling an in-line multi-beam electron gun assembly in a color television picture tube bulb of the phosphor line screen type.
In a commercial color television picture tube of the apertured mask type having a three-color viewing screen structure, the viewing screen structure is photographically printed using light centers simulative of the position of the deflection center of each of the three electron beams in the final tube. A mount assembly comprising a three-beam electron gun is subsequently installed in the tube. During the assembly of the electron gun structure in the final tube, the axis of each cathode must be oriented to coincide with the light centers used to print the viewing screen structure within a desired rotational tolerance about the central longitudinal axis of the tube. In commercial color television picture tubes using dynamic divergence circuitry, a mount assembly including an electron gun assembly having three cathodes in fixed orientation ordinarily must be positioned in the tube within three degrees of rotation. In a commercial color television picture tube using no dynamic convergence circuitry or simplified dynamic convergence circuitry, a more accurate rotational positioned in the mount assembly is usually required.
In one prior method for assembling a multi-beam electron gun structure, the alignment is accomplished by two separate assembly operations. During the mount assembly operation, the central longitudinal axis of the electron gun assembly is aligned with the stem axis and the cathode axes are rotationally aligned with the stem leads. Then, the electron gun assembly is attached to the stem leads with metal wires and ribbons to form a mount assembly. In the subsequent mount sealing operation, the preassembled mount assembly is positioned and oriented with respect to the bulb assembly and then sealed to the bulb assembly on a sealing unit. The sealing unit holds and orients the bulb assembly rotationally with respect to the major and minor axes and axially with respect to the longitudinal axis of the bulb assembly. The sealing machine also holds and orients the mount assembly axially with respect to the stem and rotationally with respect to the stem leads.
In the mount sealing operation, the mount assembly is held rotationally with the stem leads positioned within aligned holes on the sealing machine. Since the holes include a clearance for loading and the mount assembly includes assembly tolerances, the rotational alignment of mount assembly with respect to the screen structure cannot accurately be maintained. In addition, since the mount assembly is preassembled and transported to the sealing machine, the fragile wires supporting the electron gun assembly may be accidentally bent, thereby misaligning the electron gun assembly with the stem leads. This may result in an angular misalignment of the electron gun assembly when the stem leads are used to angularly align the bulb assembly and the mount assembly.
In addition, the heat used to effect mount sealing may cause a relaxation of the rotational stresses placed on the wires supporting the electron gun assembly when the electron gun assembly was initially aligned with the stem leads. This relaxation could cause further rotational misalignment. Furthermore, gauging the amount of angular rotation of the preassembled mount assembly after assembly and gauging the amount of angular rotation of the mount assembly in the assembled tube may be required to assure accurate rotational positioning of the electron beam axes with respect to the viewing screen structure in the finished tube.
In another prior method for assembling a multi-beam electron gun structure, as described in U.S. Pat. No. 3,807,006 issued to Segro et al., the alignment is accomplished by mechanically sensing the position of the electron gun assembly with respect to alignment pads on the bulb assembly. While this method is an improvement in that it obviates the necessity to align the electron gun assembly with the stem axis which is in turn aligned with respect to the bulb assembly reference pads, this method entails the necessity of physically contacting the electron gun assembly thereby introducing its own errors into the total alignment error.
Still another method for assembling a multi-beam electron gun structure comprises optically sensing the position of the electron gun assembly with respect to alignment pads on the bulb assembly. This method is an improvement over the other methods in that no physical contact is required to align the electron gun assembly with respect to these alignment pads on the bulb assembly. However, it must be noted that in all previous methods, the alignment is conducted with respect to reference pads located on the bulb assembly. It must also be noted that the optimum alignment requires aligning the electron gun assembly with the photographically printed screen on the interior surface of the faceplate panel. The introduction of an intermediate reference such as the reference pads on the bulb assembly can, and very probably does, interject additional alignment errors into the overall alignment scheme. Consequently, the most desirable method of alignment is one which aligns the electron beam apertures directly to the luminescent deposits on the screen.